const fs = require("fs");
const EventEmitter = require("events");

class MyFileReadStream extends EventEmitter {
  constructor(path, options = {}) {
    super();
    this.path = path;
    this.flags = options.flags || "r";
    this.mode = options.mode || 438;
    this.autoClose = options.autoClose || true;
    this.start = options.start || 0;
    this.end = options.end;
    this.highWaterMark = options.highWaterMark || 64 * 1024;
    this.readOffset = 0;

    this.open();

    this.on("newListener", (type) => {
      if (type === "data") {
        this.read();
      }
    });
  }
  open() {
    // 原生 open 方法来打开指定位置上的文件
    fs.open(this.path, this.flags, this.mode, (err, fd) => {
      if (err) {
        this.emit("error", err);
      }
      this.fd = fd;
      this.emit("open", fd);
    });
  }
  read() {
    if (typeof this.fd !== "number") {
      return this.once("open", this.read);
    }

    let buf = Buffer.alloc(this.highWaterMark);

    let howMuchToRead = this.end
      ? Math.min(this.end - this.readOffset + 1, this.highWaterMark)
      : this.highWaterMark;

    fs.read(
      this.fd,
      buf,
      0,
      howMuchToRead,
      this.readOffset,
      (err, readBytes) => {
        if (readBytes) {
          this.readOffset += readBytes;
          this.emit("data", buf.slice(0, readBytes));
          this.read();
        } else {
          this.emit("end");
          this.close();
        }
      }
    );
  }
  close() {
    fs.close(this.fd, () => {
      this.emit("close");
    });
  }
  pipe(ws) {
    this.on("data", (data) => {
      let flag = ws.write(data);
      if (!flag) {
        this.pause();
      }
    });
    ws.on("drain", () => {
      this.resume();
    });
  }
}

// let rs = new MyFileReadStream('test.txt', {
//   end: 7,
//   highWaterMark: 3
// })

// rs.on('data', (chunk) => {
//   console.log(chunk)
// })

module.exports = MyFileReadStream;
